Physical security systems have long used beam detectors for the detection of people and vehicles. A typical beam detector includes a light source at one end and a light sensor at the opposite end. For example, the light source may include one or more infrared LEDs emitting a beam of light toward the light sensor, which is tuned to detect the wavelength of light emitted by the light source. If a person or vehicle breaks the beam of light propagating between the light source and the light sensor, the light sensor will detect a drop in the intensity of the light reaching the sensor and thereby trigger an alarm. Beam detectors are prone to false alarms, which can occur, for example, when an animal or leaf blowing in the wind crosses the beam. The detector can also be rendered useless, intentionally or by accident, if the light source or the light sensor becomes blocked, forcing an attendant to visit the site to correct the problem. For this reason, more robust systems use a series of light sources and sensors to create a series of beams that all must be tripped before alarm-producing detection occurs.
Video surveillance technology may also be used as part of a physical security system. For example, a video surveillance camera captures images of a scene. The images may be viewed by a human observer, or the images may be transmitted to a video analytics system for detecting and tracking objects as they move through the field of view of the video camera. The video analytics system may distinguish between objects of interest and objects not of interest. For example, a human being or a vehicle may be an object of interest, but an animal or a blowing leaf may be an object not of interest. Video-based surveillance can be at least partly automated when the video analytics system includes a virtual tripwire or a region of interest (ROI) for triggering an event.
A typical virtual tripwire is a line superimposed over an image captured by a surveillance camera. An event may be triggered when the video analytics system detects an object of interest crossing the virtual tripwire. For example, the surveillance camera is positioned to capture images of a street in front of a sidewalk. A virtual tripwire may be drawn across the sidewalk, and an event would be triggered when an object of interest, such as a person, walks along the sidewalk and crosses the tripwire.
A typical ROI is defined by an area superimposed over an image captured by a surveillance camera. An event may be triggered when the video analytics system detects an object of interest moving within the area. Alternatively, an event may be triggered when the video analytics system detects an object of interest entering or leaving the area.
However, virtual tripwires and ROIs are prone to false alarms. One instance of a false alarm would be, for example, the triggering of an event when even a small portion of an object of interest crosses a tripwire or enters an ROI. Because a typical tripwire does not have a three-dimensional shape, the exact location covered by the tripwire may not always be clear. For example, a tripwire drawn across a sidewalk may appear to be on the sidewalk, but depending on the viewpoint of the camera, a person walking near the sidewalk or across the street may trigger an event if the person's head, rather than the person's feet, crosses the tripwire.
ROIs may create false alarms when used to detect people entering or leaving a doorway, because anyone walking by the front of the doorway, without passing through the doorway, may trigger an event. As another example, an ROI sized to count cars on a highway may produce false alarms caused by tree branches, shadows, headlights, or animals moving into or within the ROI. Detecting objects moving in a specific direction may also be more difficult when using an ROI for triggering an event, especially if the object wanders around and does not follow a straight path.
Moreover, the detection zone of a virtual tripwire or an ROI is static. In other words, the virtual tripwire or the ROI does not move once a user of the video surveillance system defines the tripwire or the ROI.